1. Statement of the Technical Field
The invention concerns acousto-optic devices, and more particularly, acousto-optic devices having an actuator for exciting flexural waves on an optical fiber.
2. Description of the Related Art
There are many types of fiber-optic devices known in the art which employ acoustic flexural waves. Such devices include optical switches, acousto-optic bandpass filters, acousto-optic bandstop filters, tunable optical fiber couplers, fiber-optic frequency shifters, and fiber-optic polarization analyzers. These devices typically employ acoustic flexural waves to couple light between modes of an optical fiber. A fiber-optic device's operation is often adjusted by tuning the flexural wave frequency over a large frequency range.
U.S. Pat. No. 6,021,237 to Kim et al. describes an all-fiber acousto-optic tunable filter. The acousto-optic tunable filter requires the generation of an acoustic wave and the coupling of the acoustic wave to a flexural wave on an optical fiber. This is accomplished with the use of a piezoelectric actuator and a horn that is coaxial with the optical fiber. The piezoelectric actuator produces an acoustic vibration when an electrical signal having pre-selected frequency component is applied thereon. The horn is placed between a piezoelectric actuator and the optical fiber. In this regard, the horn transmits the acoustic vibration (or an amplified variation thereof) to the optical fiber. In effect, an acoustic wave propagates along a portion of the optical fiber.
In the foregoing systems, the horn which is interposed between the piezoelectric actuator and the optical fiber performs certain functions. In particular, the horn guides a generated flexural acoustic wave toward its propagating direction. The horn also increases the amplitude of the flexural acoustic wave by concentrating the generated flexural acoustic wave at a predetermined location, which is usually the tip of the horn.
Despite the advantages of the foregoing arrangement, it suffers from certain drawbacks. For example, the fiber-optic device is designed to operate at a defined input signal frequency. As such, mechanical resonance frequencies of the horn structure produce a wide swing in the actuator's piezo efficiency as a function of an excitation frequency (i.e., an electrical signal's frequency that is applied to the actuator for inducing a particular piezoelectric effect). As a result, a continuous tuning of the excitation frequency with the fiber-optic device's acoustic or optical response has been difficult to achieve. Also, the piezoelectric actuator is affected by its temperature variations such that its performance characteristics become unstable and/or unpredictable. Further, the actuator's size and the horn's size are typically much larger than the cross-sectional size of the optical fiber. In this regard, the miniaturization of the fiber-optic device has been limited by the actuator-horn configuration.
In view of the forgoing, an acousto-optic device is needed that is designed to operate over a range of input signal frequencies. The acousto-optic device needs to provide for continuous tuning of the piezoelectric actuator's excitation frequency with the fiber-optic device's acoustic or optical response. An acousto-optic device is also needed with stable and predictable performance characteristics over a wide temperature range. Furthermore, an acousto-optic device is needed that can be miniaturized.